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The role of biologically-enhanced pore water transport
in early diagenesis: An example from carbonate
sediments in the vicinity of North Key Harbor,
Dry Tortugas National Park, Florida
by Yoko Furukawa1, Samuel J. Bentley2, Alan M. Shiller3, Dawn L. Lavoie1
and Philippe Van Cappellen4
ABSTRACT
Biologically enhanced pore water irrigation affects the course of early diagenesis in shallow
marine sediments, as illustrated here for the carbonate sediments from North Key Harbor, Dry
Tortugas National Park, Florida. Whereas macrofaunal activity at the study site extends approxi-
mately 15 cm below the water-sediment interface, measured O2microp ro les o nly show O2
penetr ation to depth s of a few mm. This apparent discrepan cy can be explaine d by consider ing the
3-D O2distribution in the burrowed sediments. Calculations based on an idealized tube model for
burrow irrigation show that measureable O2conc entration s are limited to th e immediat e v icinity of
burrows . Given the obser ved burr ow dens ity (705 615 m22), a ran domly posi tioned O2micro pro le
has a high probab ility (.90%) to fall outside the reach of radial O2diffusion from burrows. Hence,
the shallow penetration depths recorded at the site do not exclude a much deeper supply of O2in the
sediment via the burrows. Other characteristic features observed in the upper 15–20 cm of the
sedimen ts, in p articular, the abse nce of SO 422depletio n and t he presence o f su bsurfac ema xima in th e
pro les of NH41and TCO2, are also the result of pore water irrigation. These features are reproduced
by the multicomponent reactive transport model STEADYSED1. Results of the model simulations
indicate that bacterial SO422reduction is the do minant pathway of organ ic carbon degradation, but
that consumption of SO422in the sediments is compensated by its enhanced transport by irrigation.
Thus, depth pro les of SO422may be poor indicators of the importance of SO422reduction in
irrigat ed sediments.
1. Introduction
Biologically enhanced irrigation of pore water affects the spatial distribution of redox
species in shelf and coastal sediments, and consequently alters the course of early
diagen esis (Aller, 19 80; Aller and Yingst, 19 85; Boudrea u, 1997). Qu antifying irrig ation in
coastal and shelf environments is, therefore, important in order to understand early
1. Naval Research Laboratory, Sea oor Scien ces Branch, Stennis Space Center, Mississippi, 39529, U.S.A.
email: yoko.furu kawa@nrlssc.n avy.mil
2. Coastal Studies Institute, Louisiana State University, Baton Rouge, Louisiana, 70803, U.S.A.
3. Institute of Marine Sciences, Un iversity of Southern Mississi ppi, Stennis Space Cent er, Mississippi , 39529,
U.S.A.
4. Faculty of Earth Sciences, Utrecht Universit y, P.O. Box 8002 1, 3509 TA Utrecht, The Netherlands.
Journ al of Mar ine Research, 5 8, 493–522, 2000
493
diagenetic processes that control benthic exchange uxes, pollutant mobility, organic
matter prese rvation, and the forma tion of sedimentary records. The signi c ance o f
irrigation in bioturbated sediments has been demonstrated in a study by Aller (1980), in
which the depth pro les of NH41, SO422, and dissolved Si in pore water were quantita-
tively explained by a model that accounts for the activity of burrowing macrofauna.
Irrigation not only affects the depth pro les of NH41, SO422, and dissolved Si, but also
the transport of dissolved O2down the burrows. Intuitively, one suspects that deep
irrigation of O2, the most powerful oxidant in aquatic sediments, can set a course of early
diagenesis quite different from that in the absence of irrigation. However, quantitative
studies of the effect of biologically enhanced O2transport during early diagenesis are not
very abundant. Whereas the existing studies (e.g., Archer and Devol, 1992; Cai and Sayles,
1996; Fo rster and Graf, 1995; Mackin and Swid er, 1989) discu ss the increase in th e benthic
ux of O2due to irrigation, they do not explicitly discuss the effects of enhanced O2
penetration on early diagenesis.
The measurement and interpretation of depth pro les of redox species in marine
sediments have been the subject of numerous studies (e.g., Goldhaber et al., 1977;
Jørgensen, 1977; Froelich et al., 1979; Rosenfeld, 1979; Aller and Yingst, 1980; Casey and
Lasaga, 1987;Walter and Burton, 1990; Rude and Aller, 1991; Burns and Swart, 1992; and
Can eld et al., 1993). In general, SO422reduction is the major mechanism for organic
carbon (OC) remineralization on the continental shelves and in nearshore sediments
(Jørgensen, 1982; Mackin and Swider, 1989; Can eld, 1993; Tromp et al., 1995), whereas
aerobic respiration dominates degradation of OC in the deep sea (Bender and Heggie,
1984). In shallow marine depositional environments, the available O2is consumed before
the majority of the reactive OC is remineralized, prompting heterotrophic microorganisms
to utilize less efficient electron acceptors such as NO32, metal oxides, and SO422. In
support of such a scenario, O2micropro les typical ly show O2pen etration depth s of only a
few mm to sev eral cm below the water-sed iment interface (Archer and Devol, 1992; Brotas
et al., 1990; Cai and Sayles, 1996; Lohse et al., 1996; Wiltshire et al., 1996). Hence, in
nearshore sedi ments, i t is often ass umed t hat aero bic deg radation o f OC is ne gligible b elow
the upper few mm of the sediments.
A number of authors have proposed that in coastal settings the total benthic O2 ux
approaches the molecular diffusion ux to within a factor of two (e.g., Jørgensen and
Revsbech, 1985; Andersen and Helder, 1987; Mackin and Swider, 1989). Other studies,
however, have reported a signi cant enhancement of the benthic O2 ux that could be
attributed to irrigation by burrowing macrofauna. For sediments of the Washington shelf,
Archer and Devol (1992) compared direct determination of the total O2 ux across the
water-sediment interface using benthic ux chambers with diffusion uxes calculated from
pore water O2pro les. They found that bioirrigation enhances the benthic O2 ux by a
factor of three to four. They further concluded that the sediments’ low OC contents are due
to the efficient OC remineralization fueled by the enhanced O2supply. A similar c ompari-
son between measured and calculated O2 uxes in estuarine sediments found that the
494 Journal of Marine Research [58, 3
measured uxes are two to ve times greater than the calculated molecular diffusion uxes
(Hofman et al., 1991). Based on model calculations, Wang and Van Cappellen (1996)
estimate t hat bioi rrigation may a ccount for 34 to 7 8% of th e total O2e xchange ux at t hree
coastal sites in the eastern Skagerrak. Similarly, measured total O2 uxes in mud ats of
Long Island Sound were two to ve times higher than the calculated diffusion uxes
(Mackin and Swider, 1989). Thus, i rrigation po tentially may p lay a d etermining role in the
benthic O2budget of nearshore marine sediments.
This paper examines the impact of burrow irrigation on the course of early diagenesis in
the shallow-marine, carbonate environment of North Key Harbor, Dry Tortugas National
Park, Florida. The effects of irrigation on the benthic O2 ux and the distribution of O2
within the sediments are determined by combining measured O2micropro les with
calculations based on Aller’s tube model of the burrowed sediment-water interface (Aller,
1980). The results are then used to constrain the irrigation exchange function to be used in
the multicomponent reactive transport modeling. We used an existing multicomponent
reactive transport model for early diagenesis, STEADYSED1 (Van Cappellen and Wang,
1996), to examine the effect of irrigation on the OC degradation pathways and the spatial
distrib ution of pore water solut e species.
2. Study site and methods
a. Field site and sampling methods
The Dry Tortugas are located 110 km west of Key West, Florida, at the end of the Lower
Florida Keys (Fig. 1). The major constituents of the sediments are skeletal remains of
Halimeda sp., an aragonitic green alga. The sediments also contain fragments of corals,
mollusks, and foraminifers. Approximately 90 weight % of the dry bulk is composed of
carbonate minerals, with aragonite constituting nearly 50 weight %, and the rest being
composed of high-Mg and low-Mg calcite (Furukawa et al., 1997). A major source of
sediment is Halimeda fragments transported from adjacent grass beds (Bentley and
Nittrouer, 1997). The general lithology of these unconsolidated Holocene carbonate
sediments is described in Stephens et al. (1997). Active bioturbation of the sediment was
apparent in the cores collected at the sampling site. Live bioturbating organisms, mostly
spionidand capitellid polychaetes,were observeddown to approximately15 cm below the
sea oor.
The data obtained for this study were collected during May–June 1997 on board R/V
Pelican in the North Key Harbor area of the Dry Tortugas, where water depth is
approximately 10 to 12 m (Fig. 1). Fifteen cores were taken by SCUBA divers using clear
Plexig las liners. For O2micropro ling and CT scanni ng, 8.5-cm diameter liners were used,
whereas for other purposes 6-cm diameter liners were used. The diver cores penetrated to
approximately 20 cm below sea oor. Eight bottom photographs were taken by SCUBA
divers to cover 1.74 m2within 10 m of the coring site in order to quantify the density of
burrow openings. A slab core for X-radiography was collected to determine the burrow
geometry. However, the slab core penetrated only 5 cm into the sediments and did not
2000] 495
Furukawa et al.: Impact of burrow irrigation
allow us to determine the vertical extent of burrows. The latter was estimated by visual
inspection of all 15 cores and the CT scanned image of one core.
Three diver cores were sealed and refrigerated for later analysis of porosity, which took
place withi n 3 month s of sampli ng. Three div er cores were sli ced in 2 cm int ervals with in 2
Figure 1. Map of study area, North Key Harbor, Dry Tortugas National Park, Florida.
496 Journal of Marine Research [58, 3
hours of sampling, in an N2- lled glove bag. The outer 5 mm of each slice was discarded to
avoid contamination from coring. A combination pH electrode, calibrated with NBS-
traceable pH buffer solutions, was inserted directly into the sediment slice to record the pH,
before lo ading the sediment in a centrifuge bottle that had been placed in side the g love bag
before it was in ated with N2. Once capped, the centrifuge bottles were taken out of the
glove bag and centrifuged at 5,000 to 15,000 rpm for 5 to 15 minutes. The bottles were then
returned to the glove bag and the uid extract was ltered using 0.45 µm pore size Gelman
Ion Chromatography Acrodisc syringe lters. Approximately 2 to 7 ml of pore water could
be extracted from each 2 cm-thick sediment slice. Each pore water sample was acidi ed
with 50 µl 71% nitric acid and kept frozen in tightly capped high density polyethylene
bottles until analysis of sulfate, within 3.5 months of sampling.
Three other cores were similarly sliced and separated into pore water and solids within 3
hours of coring, and solids from these sections were stored frozen for later analysis of total
organic carbon (TOC) which took place within 2 months of sampling. Three more cores
were slic ed similarly and lo aded into syrin ge-type pore water sq ueezers tte d with 0 .45 µm
pore size Gelman Ion Chromatography Acrodisc syringe lters in the glove bag, and the
pore water samples extracted were analyzed for NH41and TCO2. Three more cores were
stored as backups. One 8.5 cm core was used for O2micropro ling as described below, and
the same core was later used to collect CT scan data. Two other 8.5 cm cores were stored as
backups. All fteen cores were visually inspected to determine the burrow geometry and
depth before storage or before and during slicing.
b. O2micropro les
Immediately after core retrieval on board the R/V Pelican, a Clarke-type O2microelec-
trode (Precision Measurement Engineering, Encinidas, CA) attached to a micro-
manipulator was vertically inserted into the core, starting approximately 10 mm above the
sediment-water interface, and pro ling was accomplished by lowering the electrode in
0.2 mm increments and taking measurements. The electrode was lowered until the reading
reached a steady minimum value. Eight pro les, approximately 1 cm apart from one
another, were completed within 7 hours of core collection. The electrode was calibrated
immediately before use by using surface seawater whose O2con centration was determ ined
by Winkler titration, and by assuming that the steady minimum reading below the
sediment-water interface corresponds to [O2]50.
c. Burrow characterization
Burrows were characterized using sea oor photographs taken by SCUBA divers as
described above, CT scanning, and visual inspection of cores. The 8.5 cm diameter core
was CT scanned horizontally, at every 1 mm vertical interval for the upper 5 cm of the
core, and every 1 cm for the remainder of the core. CT scanning reveals the density
distribution of each horizontal plane scanned, hence features with the similar density as
seawater were interpreted to be water- lled burrows. By stacking all horizontal data, the
three-di mensional g eometry of the bu rrows was o btained. Th e t hree-dimensio nally stacked
CT n umbers were visualized using the so ftware pack age Fortner Noes ys.
2000] 497
Furukawa et al.: Impact of burrow irrigation
The number of burrow openings was counted using the sea oor photographs. Sea oor
surface features visible in the photographs included depressions of up to 1.5 cm in diameter
and mounds of typically 1 to 4 cm in diameter. A closer visual inspection using diver cores
and CT scanned images revealed that the center of each mound was a burrow opening of
typically 1 to 3 mm in diameter. The depressions were assumed to be the collapsed feeding
pits, each of which had a burrow at the center.
d. Porosity, TOC, SO422, NH41, and TCO2
Bulk porosity was determined every 1 cm in three cores by measuring wet bulk density
and dry density using a Quantachrome Pycnometer. TOC was analyzed on solids of three
cores after centrifugation using a Leco C /S an alyzer, after eliminating i norganic carbonate
by acidi cati on. SO422was a nalyzed as tot al dissolved su lfur using Thermo Jarre ll Ash 61e
ICP. As all reduced aqueous sulfur species were driven out to the gas phase by acidi cation
right after centrifuging and ltering, the total dissolved sulfur is considered to be all in the
origina l form of SO422. NH41and TCO2were analyzed on the pore water samples from the
squeezers immedi ately after separation using a ow inject ion analysis technique (Hall and
Aller, 1992; Lustwerk and Burdige, 1995).
3. Results
a. O2micropro les
The data from O2micropro ling are shown in Figure 2. All pro les show complete
consump tion of d issolved O2within the upper 3 mm of the sediments. The pro les indicate
the existence of a diffusive boundary layer above the water-sediment interface (Jørgensen
and Revsbech, 1985). The interface is thus located below the depth at which the O2
concentration rst decreases; its position is determined to coincide with the start of the
quadratic decay of the O2pro le.
b. Burrow dist ribution and g eometry
The observed burrow density derived from the photographs is 705 (615) burrows per
square meter of seabed. This burrow density agrees well with animal counts obtained by
D’Andrea an d Lopez (19 97). These aut hors characteriz ed the benth ic macrofauna i n March
1996 in nearby Southeast Channel (see Fig. 1) and found a mean total abundance of 747.3
indivi duals per m2for the top 2 to 30 cm of the sediments. The diameters of burrows visible
in the visually inspected diver cores, CT scanned images, and X-radiography (n523) fell
between 1 and 3 mm, with an average of 2 mm. The stacked CT scanned image of the
upper 8.5 cm of the core (Fig. 3) exhibits two 4 cm-long burrows with diameters of
approximately 2 mm. A full statistical description of the orientation and vertical extent of
the burrows is not possible given the small number of burrows for which a complete data
set c ould be o btained. Howev er, based on the visu al inspec tion of visib le burrows (n523),
we estimate that, on average, the majority of burrows have an orientation near perpendicu-
lar to the water-sediment interface, and lengths of up to 15 cm. This burrow length is
supported by the 210Pb pro le of a core taken from the same location in North Key Harbor
498 Journal of Marine Research [58, 3
during the same period, which indicates rapid biogenic particle transport within the upper
15 cm of the sediment, and much slower transport below that depth (Bentley and Nittrouer,
2000). Within the 1.74 m2of the seabed surface inspected by bottom photography, on
average 16 burrows per m2of seabed (2.3% of the total number of burrows) had diameters
larger than a few mm and up to a few cm. These larger burrows are most likely created by
callian assid shrimp that burrow deeper than 3 0 cm (D’Andrea and Lopez, 1997 ).
c. Porosity, TOC, SO422, NH41, TCO2, and pH
The depth pro les of porosity, TOC, SO422, and pH are shown in Figure 4. The OC
content is low throughout the sampling depths, and there is no obvious depth-dependent
trend. The near-constant SO422concentration remains close to the value for average
seawater. Figure 5 shows the depth pro les of NH41and TCO2. Both pro les display
subsurface peaks within the upper 4–10 cm of the sediments.
4. Discussion
a. Effect of i rrigation on O2distribution
In this section, the effect of irrigation on the dissolved O2distribution is demonstrated
using calculations based on an idealized model of burrow irrigation (Aller, 1980). The
Figure 2. Oxygen micropro les measured on a diver core. The dissolved oxygen concentration
reaches zero within 2 to 3 mm below the water-sediment interface.
2000] 499
Furukawa et al.: Impact of burrow irrigation
computational steps involved are: (1) estimation of the rate of O2consump tion in t he upper
portion of the sediments; (2) incorporation of the rate term in the diffusion-reaction
equation in cylindrical coordinates to calculate the O2concentration eld; and (3)
calcula tion o f t he horizo ntally-avera ged O2depth pro le. The results of these calculations
are lat er used to constra in the o ne-dimensio nal irrigation ex change fun ction.
The reaction rate for O2consumption in the North Key Harbor sediments is determined
by matching a 1-D steady-state diffusion-reaction model to O2micropro les which are
measured sufficiently far from any burrow. Calculations presented later indicate that the
effects of radial diffusion from burrows can be neglected when the micropro le is removed
from the nearest burrow by a distance of at least 3 mm. Because of the relatively low
burrow dens ity, the p robabilit y that a m easured O2pro le will be located outside the sphere
of in uence of radial diffusion from burrows is very high, approximately 0.93. Thus, it is
most likely that at least some of the micropro les in Figure 2 re ect a balance between O2
consumption and vertical diffusion, without the effects of radial diffusion from burrows.
Figure 3 . CT scann ed imag e of the cor e use d for the oxyg en micr opro ling. Se veral burr ows of
approximately 2 mm in diameter can be observed. The core diameter and height are both 8.5 cm.
Areas with the density of bulk sediment are shown transparent, whereas those with a density close
to that of water are shown gray.
500 Journal of Marine Research [58, 3
Figure 4. Depth pro le of porosity, total organic carbon, sulfate, and pH. Sulfate concentrations
exhibit a near-constant value similar to that of bulk seawater. Total organic carbon values remain
low throu ghout the sampli ng depths .
2000] 501
Furukawa et al.: Impact of burrow irrigation
Furthermore, those pro les that are least affected by radial diffusion from burrows should
display the steepest decreases of O2with depth. Several pro les in Figure 2 satisfy that
requirement (e.g., pro les 3, 4, 6), whereas pro le 8, for instance, shows a deeper than
average O2penetration depth. In what follows, pro les 3, 4, and 6 are used to constrain the
O2consumption rate (R0).
At steady state, when vertical diffusion is the only transport mechanism, the mass
conservation equation describing the pore water pro le of O2is,
D8d2C
dx21R50 (1)
where D8is the molecul ar diffusion coefficient of O2corrected for sedimen t tortuo sity, Cis
the O2concentration and Ris the net reaction rate. For simplicity, we will assume that the
rate of consumption of O2remains constant (R5R0). For boundary conditions
C(0) 5C0(2)
C(L)50 (3)
the solution to Eq. (1) is
C(x)5C01
1
2C0
L1R0L
2D8
2
x2R0
2D8x2(4)
(Bouldi n, 1968; C ai and Sayle s, 1996) where Lis the oxygen penetration depth (Fig. 6) and
R0is the constant rate of O2consumption. From Eq. (4) a simple expression for the rate
Figure 5. Dep th pro l es of ammoni a and t otal diss olved carbo nate measure d on two div er cores . The
pro les show sub surface pe aks at 4–10 cm bel ow the water-s ediment in terface.
502 Journal of Marine Research [58, 3
term R0can b e derived by im posing that the u x of d issolved O2must be zero at x5L, that
is,
dC
dx
*
x5L
50. (5)
We then obtain
R0522D8C0
L2. (6)
Eq. (4) predicts depth pro les with a quadratic curvature. An inspection of Figure 2
indic ates this to b e true fo r the lo wer porti ons of th e measured pro les. Typically, howev er,
a layer of nite thickness separates the upper boundary of the quadratic decay portion of
the pro le from the uniform bottom water O2concentration (Fig. 6), indicating the
presence of diffusive boundary layer above the water-sediment interface (Jørgensen and
Revsbech, 1985). Thus, the concentration C0needed in Eq. (6) is not the bottom water
Figure 6. Schematic diagram de ning parameters used in the calculation of oxygen consumption
rate.
2000] 503
Furukawa et al.: Impact of burrow irrigation
concentration, but rather the concentration at the base of the boundary layer (Fig. 6). Values
of C0and Lare estimated graphically for pro les 3, 4, 6 by setting the water-sediment
interface to be where the quadratic decay of the pro le begins (Fig. 6). These values are
combined with an estimate of the sediment diffusion coefficient of O2at the measured
porosity of 62% in the upper 1 cm of the sediments. The rate of O2consumption is then
calculated with Eq. (6), as shown in Table 1. In further calculations, we use an average
consump tion rate of R052.14 M/year.
The next step is to solve the tube model equation using the rate term just calculated. In
the tube model (Aller, 1980; Aller and Yingst, 1985; Boudreau and Marinelli, 1994), the
time evolution of dissolved species concentrations results from vertical (x-direction)
diffusion, radial (r-direction) diffusion, and reaction:
C
t5D82C
x21D8
r
r
1
rC
r
2
1R0(7)
with in itial cond itions for C(x,r,t):
C(0, r, 0) 5C(x,r1, 0) 5C0(8)
C(x,r, 0) 50 (x.0, r.r1) (9)
and bou ndary conditions:
C(0, r,t)5C(x,r1,t)5C0(10)
where r1is the burrow radius (Fig. 7). In this model, the sediments are divided into
closely-packed vertical cylinders of the same geometry, by assuming identically shaped
burrows that are e qually spaced b y a dista nce 2r2(Fig. 7). At the cen ter of each cy linder is a
vertical void space that represents the burrow, and each cylinder is assigned a 2-dimen-
sional co ordinate sys tem with ve rtical and ra dial ax es. The amou nt of sedim ent that ca nnot
be accounted for by the packed cylinders (9.5 volume % of total sediments) does not
change the outcome of model calculation beyond its accuracy limit. Boundary conditions
(10) are based on the assumption of vigorous burrow ushing which maintains the solute
concen tration at the constant val ue of C0inside the burrows.
Although analytical solutions of the steady state version of Eq. (7) (C/t50) have
successfully reproduced the depth pro les of NH41, SO422, and dissolved Si in several
different sedimentary environments (Aller, 1980; Aller and Yingst, 1985), they are not
Table 1. Gra phically estim ated values of C0and L, a nd calc ulated O2consumption rate R0.
Pro le C0(µM) L(mm) R0(M/year)
3 111.5 2.00 2.18
4 140.7 2.20 2.27
6 169.5 2.60 1.96
average 140.6 2.27 2.14
504 Journal of Marine Research [58, 3
appropriate for the modeling of dissolved O2. T he con sumption rate of O2is relatively fast,
resultin g in a signi cant part of the sediment havin g an O2concentration of zero. Because
analytical solutions of diffusion-reaction equations such as Eqs. (2) and (7) only work
when the concentration has a nite value, a numerical solution was implemented. Eq. (7)
was rst split into three 1-D equati ons using the Locally 1-D M ethod of Ope rator Splitting
(Boudreau, 1997, p. 350) and each separate differential equation was solved using a nite
difference method. The Crank-Nicholson scheme (Press et al., 1992) was used to
implement the nite differencing, because the high value of R0made other numerical
schemes (i.e., fully explicit, fully implicit) unstable. The grid sizes were set to dx 5dr 5
0.01 cm, and the equations were solved as a time evolution problem (Press et al., 1992),
with a time step of dt 51024day, until steady state was reached. The latter was reached
after a few thousand time steps. The burrow geometry parameters were derived from the
burrow geometry and density observed at the study site: r150.1 cm and r252.02 cm.
Simulations were carried out assuming a constant burrow density down to a depth of 15 cm
although, in reality, not all burrows visible at the water-sediment interface (705 m22at the
study site) penetrate to 15 cm.
The cal culated dis tribution of t he O2con centration in t he surface sedime nt is shown as a
contour map on Figure 8. The calculations predict that the pore waters become anoxic a
few mm below the water-sediment interface, except in close vicinity of burrow walls.
These results are consistent with the contour maps generated by multi-component tube-
model calc ulations b y Marine lli and Bou dreau (19 96), in whic h O2, SO422, and H1systems
Figure 7. Schematic dia gram showi ng the i dealized geo metry of mo del burro ws, after Aller ( 1980).
In th e model, burro ws are ass umed to b e ident ical cylinders ,wh ere heigh t is Ban d radius i s r1. The
burrows are surrounded by identical, cylinder-shaped microenvironments with radius r2. The
cylinde rs are in a t wo-dimensio nal clos e p acking.
2000] 505
Furukawa et al.: Impact of burrow irrigation
were linked through rate expressions. The results are also consistent with data reported by
Binnerup et al. (1992) for a bioturbated estuarine sediment. Using a microelectrode, these
authors showed that O2did not persist further than 1.5 mm outside the wall of a burrow
located in an otherwise anoxic sediment.
The horizontally averaged O2pro le was calculated using the model by laterally
integrating the concentration values. The resulting pro le shows a rapid drop in the O2
concentration within the upper few mm of the sediment (Fig. 9), similar to what is observed
in the measured micropro les (Fig. 2). The ushing of the burrows with oxygenated
bottom water, however, prevents the complete disappearance of O2at depth. Acc ording to
the calculations, the average O2concentra tion stabilizes around 0 .45 µM below 3 mm (see
inset on Fig. 9).
In reality, not all burrows extend down to 15 cm. Also, they are not likely to be perfectly
ushed over their entire length. There have been relatively few studies where the chemical
composition of water inside burrows has been measured directly by microelectrodes.
Nonetheless, the existing analyses show that O2levels in burrows are indeed higher than in
the surrounding sediments (Jørgensen and Revsbech, 1985; Binnerup et al., 1992; Luther
Figure 8. Contour map of dissolved oxygen concentration calculated using the tube model. The
results indi cate that oxy gen only pe rsists in a th in laye r surrou nding the burr ows.
506 Journal of Marine Research [58, 3
et al., 1998). The existing data also suggest that ushing is not 100% efficient; that is, the
concentration of O2at depth in a burrow is typically lower than that of the bottom water
(see for example, the ne-scale O2concentration eld measured by Jørgensen and
Revsbech, 1985). The multi-component tube model results by Marinelli and Boudreau
(1996) a lso show a signi cantly l ower O2conce ntration within a burrow when discontinu-
ous irrigation is implemented.
Another assumption underlying the results in Figures 8 and 9 is that the rate of O2
consumption outside a burrow wall is constant and equal to the value extracted from the
vertical O2micropro les collected in the upper few mm of the sediments. Marinelli and
Boudreau (1996) attributed the disagreement between their modeled and laboratory-
measured O2distrib utions to th e poss ible difference i n O2consumption processes and thus
the rates between the burrow walls and water-sediment interface. One would, therefore,
expect the calculated O2distribution to be most reliable near the water-sediment interface
where the O2consumption rate was calibrated, and where the O2consumption inside a
burrow approaches the bottom water value. The horizontally-averaged O2pro le is also
most reliable in the upper part of the sediments where the number of burrows is close to
that of burrow openings counted at the water-sediment interface.
Despite the uncertainties associated with the model assumptions, the calculations in
Figures 8 and 9 emphasize that deep O2penetration in a sediment may not be apparent in
measured O2micropro les. A microelectrode randomly inserted in irrigated nearshore
sediments will most likely show a complete disappearance of pore water O2within the
upper few mm, unless it happens to intersect a burrow or come to within a very short
Figure 9. Hor izontally ave raged oxy gen pro le cal culated using th e model. The dis solved ox ygen
concen tration at th e depths wher e th e effec t of vertica l diffusi on is ne gligible (x.0.3 cm) is very
small (0.5 31023mM) but nite.
2000] 507
Furukawa et al.: Impact of burrow irrigation
distance of a burrow wall (Fig. 8). Thus, the results presented here imply that the shallow
O2penetration depths recorded by us (Fig. 2) and others in coastal marine sediments do not
necessarily exclude a supply of O2to greater depths where it may sustain aerobic
respiration and cause the oxidation of reduced by-products of anaerobic degradation of
organic matter (Wang and Van Cap pellen, 1996).
b. Benthic O2 u x
For each model cylinder whose geometry is given in Figure 7, the vertical ux of O2at
the water-sediment interface is given by
DFx52p w D8e21
22r
C
x
*
x50
dr (11)
where wrepresents porosity, and the radial ux of O2at the burrow wall is given by
DFr5e0
2p
1
wD8e0
BC
r
*
r5r1
dx
2
r1du. (12)
Because the pore water O2concentration gradient is not parallel to the water-sediment
interface and burrow wall especially near (x,r)5(0.0, 0.1) (see Fig. 8), solutions for the
above equations cannot be numerically estimated by simply differencing C0and the
concentrations at the rst nodes within the interface (Boudreau and Taylor, 1989;
Boudreau, 1997, p. 360). Fluxes are thus calculated using the 2-D interpolation procedure
described in Evans and Gourlay (1977). In this procedure, C(x,r) for each grid along the
water-sediment interface and burrow wall is rst expressed as a function of xand rusing
bilinear interpolating equations (Press et al., 1992). Subsequently (C/x)x50and (C/
r)r5r1are analytically determined and integrated along the water-sediment interface (for
(C/x)x50) and burrow wall (for (C/r)r5r1) according to Eqs. (11) and (12).
The vertical (water-sediment interface) and radial (burrow wall) uxes for each model
cylinder (i.e., r150.1 cm, r252.02 cm, B515 cm) are c alculated as ab ove to be: DFx5
3.22 mmoles year21; and DFr53.48 mmoles year21. There are 0 .0705 burrows in a sq uare
centimeter of the seabed, thus the vertical, radial, and total uxes for the unit area (1 cm2)
of seabed are: Fx52.27 31024moles cm22year21; and Fr52.45 31024moles cm22
year21, and SF54.72 31024moles cm22year21. The burrows increase oxygen ux by
more than 100%. In comparison, the model burrows occupy only 0.2% of the total
sediment volume within the upper 15 cm of the sediments.
The above calculated enhancement of the benthic O2 ux represents an upper limit,
because it is based on a perfect ushing of the burrows over their entire length. The
calculations also assume that all burrows extend down to 15 cm below the water-sediment
interface. As shown in Figure 10, however, even if bottom water O2concentrations only
persist over a few centimeters along active burrows, a signi cant effect of irrigation on the
sediment O2budget can be expected. Furthermore, many coastal sites visited by previous
studies exhibit macrofaunal population densities higher than observed in this study site.
508 Journal of Marine Research [58, 3
For instance, the population density of macrofauna larger than 0.5 mm was greater than
21,000 individuals/m2during the spring bloom of 1993 in Long Island Sound (Gerino et
al., 1998). There were more than 2,300 individuals/m2of spionid polychaete in April,
1992, in Massachusetts Bay (Wheatcroft et al., 1994). Blair et al. (1996) counted 12,000
individuals/m2in cores coll ected from the cont inental slo pe off Cape Hatt eras, an d Smet hie
et al. (1981) observed 3400 individuals/m2on the Washington continental shelf off the
Colombia River. According to the ux calculation results shown in Figure 10, these high
macrofaunal population densities, typical of many coastal sediments, can signi cantly
increase the O2uptake by the sediments, particularly where the dominant macrofauna
actively ush their burrows to depths exceeding 1 to 2 cm.
c. Effect of irrigati on on OC diag enesis an d pore wate r solut e pro les
Because of burrow irrigation, some dissolved O2bypasses the zone of very reactive OC
at the water-sediment interface and reacts at depth in the sediment. Irrigation also increases
the uxes of other oxidized solute species, such as SO422and NO32, into the sediment. By
enhancing the supply of oxidants, irrigation signi cantly affects the degradation of organic
Figure 10. Increase of benthic oxygen ux due to burrow irrigation as calculated by Eqs. (11) and
(12). The vertical model burrows have a radius of 1 mm, and densities of 705, 5,000 and 10,000
burrows/m2. The active burrow length refers to the depth to which the oxygen concentrationinside
the burrow is near the bottom water value.
2000] 509
Furukawa et al.: Impact of burrow irrigation
matter (OM) and the redox conditions below the upper few mm of sediment. The effects of
irrigation on the early diagenesis of North Key Harbor sediments are explored here using
STEADYSED1, a multicomponent reactive transport computer code (Van Cappellen and
Wang, 1996; Wang and Van Ca ppellen, 1996).
STEADYSED1 numerically simulates reaction and transport processes of C, O, N, S,
Fe, and Mn in steady state early diagenetic systems. The solids and adsorbed species are
transported via sediment burial advection and bioturbation, where the latter is treated as
one-dimensional diffusion, parameterized by a depth-dependent biodiffusion coefficient
Dbx (Berner, 1980). Solute transport is treated as a combination of molecular diffusion
perpendicular to the water-sediment interface (x(D8·xC(x))), se diment burial advection,
and pore water irrigation. The latter is described by a one-dimensional nonlocal transport
model where the solute transfer rate at any given depth is given by ax(C02C(x)), axbeing
the depth-dependent irrigation exchange coefficient (Emerson, et al., 1984; Boudreau,
1984). Th e molecu lar diffusion co efficient, D8, is spe cies dep endent whereas axand Dbx are
treated to be species independent.
Chemical species and reactions included in STEADYSED1 are listed in the Appendix.
Acid dissociation reactions in the dissolved carbonate and sul de systems (not shown in
the Appendix) and adsorption processes are assumed to be at equilibrium. The other
reactions are represented by kinetic expressions in the model. The total rate of organic
carbon oxidation is b roken down i nto the con tributions o f aero bic resp iration, d enitri ca-
tion, d issimilatory Mn (IV) reduc tion, dissimila tory Fe(III) reduction, sulfate reduction and
methano genesis. The respirato ry path ways are assumed to fol low Mo nod kine tics. Crit ical
parameters in the rate expressions are a set of limiting concentrations for the terminal
electron acceptors. These parameters account for substrate limitation and control competi-
tion among the various organic matter degradation pathways (Van Cappellen and Wang,
1996). The rates of the secondary redox reactions are described by bimolecular kinetic
expressio ns, R5k[ox][red], where [ox] is the concentrationof the oxidant, [red] that of the
reductan t. The ra tes o f the min eral diss olution an d prec ipitation reac tions are a ssumed to b e
linear functions of the degree of saturation. STEADYSED1 supplies default values for the
various kinetic and equilibrium parameters needed in the reaction model. These values,
however, can be modi ed by the user via an interactive input interface.
Where available, values for input parameters measured at the study site are used in the
STEADYSED1 si mulations. Oth er paramete rs are es timated from gl obal empiric al correla-
tions, which relate properties of early diagenetic systems to a ‘‘master’’ variable, usually
sedimentation rate, water depth, or deposition ux of OC. Some parameters are determined
through educated guesses, based on the characteristics of the depositional environment.
The input parameters for the simulations are summarized in Table 2. No attempt is made to
nd an optimal t between calculated and measured depth pro les. Rather STEADYSED1
is used as a predictive simulator of the behavior of a model sediment with properties
approaching that of the actual sediment.
The sedimentation rate at the study s ite, determined from 210Pb pro les, is 0.42 cm yr21
510 Journal of Marine Research [58, 3
(Bentley and Nittrouer, 2000). The 210Pb pro les also yield a maximum estimate of the
particle mixing coefficient, Db, in the top sediment layer of 130 cm2yr21(Bentley and
Nittroue r, 2000 ). Although the e xact d epth distribu tion of Dbis not known, it is expected to
decrease with depth (Boudreau, 1986). Therefore, we use the following available option in
STEADYSED1 to represent the depth pro le of Db,
Db5D0exp 52(x/xb)26(13)
(Christen sen, 198 2) where D05130 cm2yr21. T he adju stable charac teristic dept h, xb, is set
equal to 5 cm. This results in a Dbpro le where particle mixing is most intense within the
upper 10 cm of sediment and negligible below 15 cm, in agreement with the 210Pb data
collected at the site (Bentley and Nittrouer, 2000). This model Dbpro le is shown in
Figure 11A.
An estimate of the nonlocal irrigation exchange coefficient, a, of O2in the upper cm of
the sediment is obtained from the modeled O2distribution around the burrows (Fig. 8).
Boudre au (1984) ha s shown t hat, und er given con ditions, the i dealized tu be-model (Fig . 7)
and the non-local exchange model used in STEADYSED1 are equivalent, and can be
related via the following equation:
a 5 2D8r1
(r2
22r1
2)(r2r1)(14)
where rcorresponds to the distance from the burrow axis to the point where the
concentration of dissolved O2equals the horizontally integrated value. Value of rcan
Table 2. I nput parameters used in the STEADYSED1 simulations.
Parameters Values Methods
Temperature 28.5 (C) Bottom water temperature
measured by CTD
Salinity 36.3 (‰) Bottom water salinity
measured by CTD
Sediment ation Rate 0.42 (cm/ye ar) Bentley an d Nittroue r (2000)
Bulk dry sediment density 2.74 (g/cm3) Pychnometer
Porosity 58 (%) Pychnometer
Sediment particle mixing co-
efficient (Db)
Db5130 exp 52(x/5)26
(cm2/year)
D0from Bentley and
Nittrouer (2000)
Irriga tion coefficie nt (a)a(x)5 a 0exp 52 a 1x6(yr21) Eq. (15); see text
Bottom water O20.14 (mM) Measured
Deposit ion ux of Mn o xides 1.0 31026(mole/cm2/year) Estimated
Deposition ux of Fe oxides 5.0 31026(mole/cm2/year) Estimated
Organic C d eposition ux 5.7 31024(mole/cm2/year) See text
% meta bolizable C 65 Tromp et al. (1995)
C deg radation ra te con stant 1.7 (yea r21) See text
Red el d ratio ( C, N, P) 106, 9, 1 Wang an d Van Cappellen
(1996)
2000] 511
Furukawa et al.: Impact of burrow irrigation
therefore be obtained by comparing the horizontally averaged O2concentration pro le
(Fig. 9) with the 2-D concentration eld (Fig. 8). Except for the uppermost few millime-
ters, ris found to be 0.26 cm (within the depth interval 0 20.33 cm, where rvaries
between 0.47 20.26 cm). The sediment diffusion coefficient of O2, adjusted for sediment
temperature and porosity, is 311 cm2yr21. Hence Eq. (14) yields a value for aof 96 yr21.
This value falls within the range reported for nearshore and continental shelf sediments
(e.g., Emerson et al., 1984; Christensen et al., 1987; Martin and Banta, 1992; Wang and
Van Cappellen, 1996).
The depth distribution of ais not known for the North Key Harbor sediments. In the
idealized scenario considered in Section 4a, where all burrows are 15 cm deep and 100%
irrigated , the values of rand aremain co nstant down to 15 cm. However, it is exp ected that
the composition of burrow water deviates progressively from that of overlying water with
increasing depth. Furthermore, not all burrows extend to 15 cm below water-sediment
interface. Consequently, the value of acalculated above (96 yr21) is only reliable within
the topmost sediment layer. To describe the depth dependence of a, we consider a simple
exponential decay:
a(x)5 a 0exp 52 a 1x6(15)
where a0596 yr21and a1is the decay constant. Eq. (15) has been used successfully to t
data from a numbe r of nearsho re marine env ironments (Ma rtin and B anta, 19 92; Wang an d
Van Cappellen, 1996). In the simulations presented below, the same irrigation coefficient
Figure 11. (A) Depth p ro le of the biod iffusion coefficient (Db) used in the STEADYSED1
simulati ons. (B) Depth pro les of th e irrigation excha nge coefficient (a) used in the STEADY-
SED1 s imulations. Th e pro les are i n the form of a 5 a 0exp (2 a 1x) where a0596 (yr21) and
a150.2 or 0.5 (cm21), or in the form of a 5 96 (yr21) (for x,1 5 cm) and a 5 0 ( for x.15 cm).
512 Journal of Marine Research [58, 3
a(x) is applied to all solute species. The depth distributionsof the a(x) functions used in the
simulations are shown in Figure 11B.
The deposition of ux of OC is estimated as follows. Below a depth of 20 cm, sediment
mixing is negligible and solid sediment transport is exclusively via sediment advection.
The concentration of total organic carbon (TOC) in the depth interval 15–20 cm is on the
order of 0.5 wt% (Fig. 4). Hence, assuming steady state compaction, the measured values
of TOC, porosi ty, solid sediment density and sedimentation rate are combined to calculate
a burial rate of OC below the zone of early diagenesis (upper 10–20 cm) of 2.01 3
1024moles cm22yr21. For sed iments with a ccumulatio n rates on t he order of 0. 42 c m yr21,
the burial efficiency (i.e., the ratio of burial ux to deposition ux) of OC is in the range of
20–50% (Henrichs and Reeburgh, 1987; Tromp et al., 19 95). An a verage buria l efficiency
of 35% therefore yields an OC deposition ux of about 5.7 31024moles cm22yr21, which
is the value used in the simulations. It agrees well with estimates obtained from global
empirical correlations between the OC deposition ux and sedimentation rate (Henrichs
and Reeburgh, 1987; Tromp et al., 1995), or water depth (Middelburg et al., 1997). These
correlations predict values between 5.5 and 11.8 31024moles cm22yr21. The rst-order
reaction rat e decay c onstant, k, for OM d egrading in th e bioturbat ed zone of the sed iment is
calculated from the empirical correlation between kand sedimentation rate proposed by
Tromp et al. (1995).
In the absence of a nearby detrital source, the supply of reactive iron and manganese
(hydr)oxides to the sediments in North Key Harbor is likely to be fairly small. Because no
data are available to directly constrain the deposition uxes of these reactive phases, they
are assumed to be one order of magnitude lower than those obtained for a nearshore
siliciclastic sediment with a similar sedimentation rate (Wang and Van Cappellen, 1996).
The bottom water concentrations imposed in the simulations are those measured at the site.
All reaction parameters (i.e., equilibrium constants, rate constants, and limiting concentra-
tions of terminal electron acceptors for Monod kinetics) are the default values provided by
STEADYSED1 (for complete discussion on how these were obtained, see Van Cappellen
and Wang, 1996; Wang and Van Cappellen, 1996). The exception is the rate constant for
nitri cation which was revised upward to 0.5 31028M21yr21, a value which falls within
the range reported by Wang and Van Cappellen (1996).
Calculated pore water pro les for a number of different depth distributions of the
irrigation exchange coefficient are presented in Figure 12. The corresponding depth
distributions of ain Figure 11B are generated by varying the decay constant a1(Eq. (15)).
As can b e seen, when signi cant irrigation intensity persists to depth of 15 cm
(a150.2 cm21), the simulations reproduce the features of the pore water pro les observed
at the study site, namely, the lack of depletion of SO422, the subsurface maxima of NH41
and TCO2, and the rapid decrease of pH within the rst cm, followed by near constant
values (Figs. 4 and 5). The simulations also predict little change with depth of the
concentration of OC (Fig. 13). This re ects the fairly intense bioturbation which rapidly
mixes organic ma tter do wnwards a nd homo genizes se diment co mposition. Th us, al though
2000] 513
Furukawa et al.: Impact of burrow irrigation
Figure 12. Depth pro les of sulfate, ammonia, total dissolved carbonate, and pH calculated by
STEADYSED1 simulations using the parameter values shown in Table 2 and Figure 11. Note that
when signi ca nt amoun t o f irrig ation is provid ed to 15 cm (i.e. , a150.2 cm21), simulations
reproduce the pore water pro le characteristics observed at the study site.
514 Journal of Marine Research [58, 3
about 65% of the deposited organic carbon is oxidized within the upper 15 cm, the
degradation process is not recorded by the cm-scale TOC depth pro les. The model-
predict ed distrib ution of OM degrad ation path ways is sho wn in F igure 1 4.
In the simulations performed with the decay constant value of a150.2 cm21, solute
transpo rt by irrigation takes place over the entire depth range where there is signi cant OM
degradation (compare Figs. 11B and 14). Below 6.5 cm, the in ux of SO422from burrows
exceeds the rate of bacterial SO422reduction, hence causing the SO422concentration to
return to near-bottom water values (Figs. 4 and 12). Similarly, the decreasing concentra-
tions of NH41and TCO2in t he lower p ortions of th e cores (Figs. 5 an d 12) are exp lained by
their faster removal via irrigation, compared to their production by OM decay. The depth
pro le of the irrigation exchange coefficient for a150.2 cm21is consistent with the
observe d burrow di stribution at the study site. Because many burrows are found to extend
to 15 cm below the water-sediment interface, it is reasonable to expect ushing by infauna
to affect pore water composition to at least that depth.
The integrated rates of the various OM degradation pathways are compared in Table 3,
Figure 13. Depth pro les o f total organi c carbon (weight %) calculated by addin g 0.5 wt%
(estimate d amount o f iner t organi c carbon) to the mo del-derive d depth di stribution of metab oliz-
able organiccarbon.
2000] 515
Furukawa et al.: Impact of burrow irrigation
for the simulations with a0596 yr21and a150.2 cm21. According to the calculations,
SO422reduction is the major pathway for OM degradation, accounting for about 80% of
the total OC oxidized. This dominant role of SO422reduction in North Key Harbor
sediments contrasts with the pore water SO422pro le which shows little depth-dependent
decrease. Similar situations have been studied in Washington continental shelf (Chris-
tensen et al., 1984) and Amazon shelf (Aller and Blair, 1996) where observed SO422
Figure 14. Model-derived depth distribution of the organic carbon oxidation pathways. Note that
sulfate redu ction is the do minant pathwa y below 1 cm.
Table 3. Comparison of organic carbon degradation pathways in upper 15 cm of the sediments
modeled by STEADYSED1, for the si mulation with a0596 (yr21) and a15 2 0.2 (cm21).
OC deg radation ra te
(31024moles/cm2/yr)
OC deg radation ra te by :
Aerobic respiration 0.5 (13%)
NO3reduction 0.2 (5%)
Mn & Fe reduction 0.0 (0%)
SO4reduction 3.1 (82%)
516 Journal of Marine Research [58, 3
pro les could be explained by invoking intense pore water irrigation and reoxidation of
sulfur species due to biological and physical rewo rking.
The effects of irrigation on early diagenesis are further assessed through sensitivity
analyses where a0and a1are varied (Table 4). The calculations show that SO422reduction
remains th e main p athway for OM d egradation un der widely variab le irrigation c onditions.
As exp ected, the co ntribution o f aerob ic respirati on to tot al OM deg radation inc reases with
increasing irrigation intensity. However, the results in Table 4 indicate that, under widely
variable irrigation conditions, a large fraction of the benthic O2 ux is used to re-oxidize
reduced byproducts of OM deg radation (e.g., NH41, Fe(II), Mn(II), H2S, iron sul des).
5. Conclusions
The geochemical data collected in the carbonate sediments of North Key Harbor
illust rate seemi ngly contrad ictory early diagenetic pictures. The O2penetration depths o f a
few mm and the abundant benthic macrofauna are indicative of a large supply of reactive
OM to the water-sediment interface. On the other hand, the lack of depth-dependent
chang es in the pro les of pore water SO422and TOC in the upper 20 cm of sediment do not
immedia tely sugg est any signi ca nt degrad ation of OM. A consi stent expl anation emerges,
howeve r, when the role s of pore water irrigati on and biotu rbation are acco unted for.
Burrow irriga tion crea tes a pat hway for the in troduction o f O2and oth er oxidan ts (SO422
and NO32) at depth in th e sed iment. Ca lculation s with an ideal ized tub e-model for b urrows
show that measurable O2concentrations are restricted to the immediate vicinity of the
burrow walls. Hence, pro les measured with microelectrodes will provide clear evidence
for deep O2penetration by irrigation only when they directly intersect burrows. At the site
studied, the likelihood of this occurrence is very small, because the burrow openings
occupy only about 0.2% of the total sediment surface area. The very shallow O2penetration
depths measured in North Key Harbor therefore do not exclude a supply of O2below the
water-sediment interface, where O2affects the course of early diagenesis.
Tale 4. Sensit ivity Analys is Resu lts.
a0
(yr21)
a1
(cm21)
% OC de gradation by : % O2
consume d by
aerobic
respiration
Aerobic
respiration
NO3
reduction
Mn & Fe
reduction
SO4
reduction
Deep 0 23 7 3 67 67
Irrigation 0.01 22 7 3 68 67
96 0.1
0.2
16
14
5
4
2
1
77
81
62
64
Shallow 0.5 12 4 0 84 66
Irrigation 1.0 11 4 0 85 64
300 0.2 24 7 4 65 66
10 0.2 12 3 0 85 65
2000] 517
Furukawa et al.: Impact of burrow irrigation
Signi cant irrigation to depths of at least 15 cm explains the observed pore water
pro les of SO422, NH41and TCO2. Calculations with STEADYSED1 indicate that
bacterial SO422reduction is the main pathway of OM degradation below the water-
sediment interface. Efficient irrigation, however, replenishes the pore water SO422reser-
voir, hence little depletion of the SO422concentration with depth is observed. Similarly,
efficient irrigation in the lower portions of the sediment cores is responsible for the
subsurface maxima seen in the pro les of NH41and TCO2. The importance of OM
degradation in the upper 15 cm of sediment is not re ected in the TOC concentrations,
because of the rapid remineralization and efficient infaunal mixing of the sediment.
The simulations presented in this paper demonstrate that the multicomponent reactive
transport model STEADYSED1 offers a useful interpretative framework in which to
analyze chemical pro les in sediments. A number of similar comprehensive models for
early diagenesis are now available (Boudreau, 1996; Soetaert et al., 1996). Because these
models simultaneously account for the multiple reaction and transport couplings between
chemical species, they are particularly useful in predicting the collective behavior of early
diagenetic systems. In the case studied here, the proposed interpretation of the pore water
data in terms of the irrigation regime is strengthened by the ability of the model to
simultaneously reproduce the features of all measured depth pro les (pH, SO422, NH41,
TCO2and TOC). The results also show that global empirical correlations yield reliable
information that enhances existi ng early diagenetic data sets.
Acknowledgments. We thank K. Mohanty for making CT scan equipment available to us, and
Jinchun Yuan, K. Stephens, K. Briggs and the NRL Dive Team for their assistance in sampling,
sample pr ocessing, an d an alysis. The c aptain an d cr ew of R/V Pelican were most helpful in making
the on-board sampling and sample processing a success. This study was funded by ONR 322GG
(P.E. No. 060 1153N) and b y NRL (EBBL-SRP).
APPENDIX
Biogeochemical reactions included in STEADYSED1 (from Van Cappellen and Wang
(1996) and Wang and Van Cappellen (1996))
Primary redox reactions:
(CH2O)x(NH3)y(H3PO4)z1(x 12y)O21(y 12z)HCO32®(x 1y12z)CO21
yNO321zHPO4221(x 12y 12z)H2O
(CH2O)x(NH3)y(H3PO4)z1((4x 13y)/5)NO32®((2x 14y)/5)N21((x 23y 1
10x)/5)CO21((4x 13y 210z)/5)HCO321zHPO4221((3x 16y 110z)/5)H2O
(CH2O)x(NH3)y(H3PO4)z12xMnO21(3x 1y22z)CO21(x 1y22z)H2O®
2xMn211(4x 1y22z)HCO321yNH411zHPO422
(CH2O)x(NH3)y(H3PO4)z14xFe(OH)31(7x 1y22z)CO2®4xFe211
(8x 1y22z)HCO321yNH411zHPO4221(3x 2y12z)H2O
(CH2O)x(NH3)y(H3PO4)z1(x/2)SO4221(y 22z)CO21(y 22z)H2O®
(x/2)H2S1(x 1y22z)HCO321yNH411zHPO422
(CH2O)x(NH3)y(H3PO4)z1(y 22z)H2O®(x/2)CH41((x 22y 14z)/2)CO21
(y 22z)HCO321yNH411zHPO422
518 Journal of Marine Research [58, 3
REFERENCES
Aller, R. C. 1980. Quantifying solute distributions in the bioturbated zone of marine sediments by
de ning an average microenvironment.Geochi m. Cosmoc him. Acta, 44, 1955–1965.
Aller, R. A. and N. E. Blai r. 1996. Sulfu r diagenesisand burial on the Amazon shelf: Major c ontrol by
physica l sedimen tation pro cesses. Geo -Marine Lett., 16, 3–10.
Aller, R. C. and J. Y. Yingst. 1980. Relationships between microbial distributions and the anaerobic
decomposition of organic matter in surface sediments of Long Island Sound, USA. Mar. Biol., 56,
29–42.
—— 1985. Effects of the marine deposit-feeders Het eromastus lif ormis (Polychaeta), Macoma
balthica (Bivalvia), and Tellina texana (Bivalvia) on averaged sedimentary solute transport,
reactio n rates, an d microbi al distributi ons. J. Mar. Res., 43, 615–645.
Andersen, F. O. and W. Helder. 1987. Comparison of oxygen microgradients, oxygen ux rates and
electro n transport sy stem activity in coastal mari ne sedimen t. Mar. Ecol. Prog. Ser., 37, 259–264.
Archer, D. and A. Devol. 1992. Benthic oxygen uxes on the Washington shelf and slope: A
comparison of in situ microelectrode and chamber ux measurements. Limnol. Oceanogr., 37,
614–629.
Bender, M. L. and D. T. Hegg ie. 1984. Fate o f organic carbon reaching the deep sea oor: A status
report. Ge ochim. Cosmoch im. Acta, 48, 977–986.
Bentley, S. J. and C. A. N ittrouer. 1997. Biogeni c in uen ces on the f ormation of sedime ntary fabric in
a ne-graine d carbonate-shelf enviro nment: Dry Tortugas, Flo rida Keys. Geo-Marine Lett., 17,
268–275.
—— 2000. Production and accumulation of bioclastic muds in a carbonate-platform setting: Dry
Tortugas, Florida. J. Sed. Res. (submitted).
APPENDIX (continued)
Secondaryredox reactions:
Mn211(1/2)O212HCO32®MnO212CO21H2O
S2Mn11(1/2)O21HCO32®S2H01MnO21CO2
Fe211(1/4)O212HCO321(1/2)H2O®Fe(OH)312CO2
S2Fe11(1/4)O21HCO321(3/2)H2O®S2H01Fe(OH)31CO2
2Fe211MnO212HCO3212H2O®2Fe(OH)31Mn2112CO2
NH4112O212HCO32®NO3212CO213H2O
H2S12O212HCO32®SO42212CO212H2O
H2S12CO21MnO2®Mn211S012HCO32
H2S14CO212Fe(OH)3®2Fe211S014HCO3212H2O
FeS 12O2®Fe211SO422
CH412O2®CO212H2O
CH41CO21SO422®2HCO321H2S
Adsorpt ion reactions:
NH41NH41(ads)
S2H01Mn211HCO32S2Mn11CO21H2O
S2H01Fe211HCO32S2Fe11CO21H2O
Precipitat ion and disso lution reaction s:
Mn2112HCO32MnCO31CO21H2O
Fe2112HCO32FeCO31CO21H2O
Fe2112HCO321H2S FeS 12CO212H2O
2000] 519
Furukawa et al.: Impact of burrow irrigation
Berner, R. A. 1980. Early Diagenesis: A Mathematical Approach. Princeton University Press,
Princeto n, NJ 241 pp .
Binnerup, S. J., K. Jensen, N. P. Revsbech, M. H. Jensen and J. Sorensen. 1992. Denitri cation,
dissimilatory reduction of nitrate to ammonium, and nitri cation in a bioturbated estuarine
sediment as measured with N-15 and microsensor techniques. Appl. Environ. Microbiol. 58,
303–313.
Blair, N. E., L. A. Levin, D. J. DeMaster and G. Plaia. 1996. The short-term fate of fresh algal carbon
in conti nental slo pe se diments. Limnol. Ocea nogr., 41, 1208–1219.
Boudreau, B. P. 1984. On the equivalence of nonlocal and radial-diffusion models for porewater
irrigat ion. J. Mar. Res., 42, 731–735.
—— 1986. Mathematics of tracer mixing in sediments: I Spatially-dependent, diffusive mixing.
Amer. J. Sci., 286, 161–198.
—— 1996. A method-of-lines code for carbon and nutrient diagenesis in aquatic sediments.
Computer s & Geos ciences, 22, 479–496.
—— 1997 . Diagene tic Models and Their Imple mentation s.Spri nger-Verlag, Hei delberg, 41 4 pp.
Boudrea u, B. P. an d R. L. Marin elli. 1994 .A modelin g study of discon tinuous biolo gical irriga tion. J.
Mar. Res., 52, 947–968.
Boudreau, B. P. and R. J. Taylor. 1989. A theoretical study of diagenetic concentration elds near
manganesenodules at the sediment-waterinterface.J. Ge ophys. Res., 94, 2124–2136.
Bouldin, D. R. 1968. Models for describing the diffusion of oxygen and other mobile constituents
across the mud- water inte rface. J. Ecol., 56, 77–87.
Brotas, V., A. Amo rimferreira, C. Vale, an d F. Cat ario. 1 990. Oxy gen pr o les in inte rtidal sedim ents
of Ria Formo sa (S Port ugal). Hydrobiologia,207, 123–129.
Burns, S. J. and P. K. Swart. 1992. Diagenetic processes i n Holocene carbonate s ediments: Florida
Bay mudba nks and i slands. Sedime ntol., 39, 285–304.
Cai, W. J. and F. L. Sayles. 1996. Oxygen penetration depths and uxes in marine sediments. Mar.
Chem., 52, 123–131.
Can eld, D. E. 1993. Organic matter oxidation in marine sediments, in Interac tions of C, N, a nd S.
Biogeochemical Cycles and Global Change, R. Wollast, F. T. Mackenzie and L. Chou, eds., NATO
ASI Series v.14, 333 –363.
Can eld , D. E., B. Th amdrup a nd J. W. Han sen. 1993. The anaerobic deg radation of o rganic-matter
in Danish coastal sediments—iron reduction, manganese reduction, and sulfate reduction. Geo-
chim. Cosmoc him. Acta, 57, 3867–3883.
Casey, W. H. and A. C. Lasaga. 1987. Modeling solute transport and sulfate reduction in marsh
sedimen ts. Geochim. Cosmoc him. Acta, 51, 1109–1120.
Christensen, E. R. 1982. A model for radionuclides in sediments in uenced by mixing and
compac tion. J. Geop hys. Res., 87C1, 566–572.
Christensen, E. R., A. H. Devol and W. M. Smethie. 1984. Biological enhancement of solute
exchange between sediments and bottom water on the Washington continental shelf. Cont. Shelf
Res., 3, 9–23.
Christensen, E. R., W. M. Smethie and A. H. Devol. 1987. Benthic nutrient regeneration and
denitri cation on the Washingto n contin ental shel f. Deep -Sea Re s., 34, 1027–1047.
D’Andrea, A. F. and G. R. Lopez. 1997. Benthic macrofauna in a shallow water carbonate sediments:
major biotu rbators at th e Dry Tortugas. Geo-Mar ine Lett., 17, 276–282.
Emerson, S., R. Jahnke and D. Heggie. 1984. Sediment-water exchange in shallow water estuarine
sedimen ts. J. Mar. Res., 42, 709–730.
Evans, N. T. S. and A. R. Gourlay. 1977. Solution of a 2-dimensional time-dependent diffusion
problem concerned with oxygen-metabolism in tissues. J. Inst. Math. Appl., 19, 239–251.
520 Journal of Marine Research [58, 3
Forster, S. and G. Graf. 1995. Impact of irrigation on oxygen ux into the sediment—intermittent
pumping by callianassa-subterranea and piston-pumping by lanice-conchilega. Mar. Biol., 123,
335–346.
Froelich, P. N., G. P. Klinkhammer, M. L. Bender, N. Luedtke, G. R. Heath, D. Cullen, P. Dauphin,
D. Hammond, B. Hartman and V. Maynard. 1979. Early oxidation of organic matter in pelagic
sediment s of the e astern eq uatorial Atlanti c; subox ic diagene sis. Geochim. Cos mochim. Acta, 43,
1075–1090.
Furukawa , Y., D. Lav oie an d K. Stephens. 19 97. Effect of biog eochemical diagenesis on sediment
fabric in shallow marine carbonate sediments near the Dry Tortugas, Florida. Geo-Marine Lett.,
17, 283–290.
Gerino, M., R. Aller, C. Lee, J. K. Cochran, J. Aller, M. Green and D. Hirschberg. 1998. Comparison
of different tracers and methods used to quantify bioturbation during a spring bloom: 234-
Thorium, lumin ophore s and chloro phyll a. Estuar. Coast. Shelf. Sci., 46, 531–547.
Goldhaber, M. B., R. C. Aller, J. K. Cochran, J. K. Rosen eld, C. S. Martens and R. A. Berner. 1977.
Sulfate reduction, diffusion, and bioturbation in Long Island Sound sediments: Report of the
FOAM grou p. Amer. J. Sci., 277, 193–237.
Hall, Per O. J. and R. C. Aller. 1992. Rapid, small-volume, ow injection analysis for SCO2and
NH41in marine an d fresh waters. Limnol . Oceanog r., 37, 1113–1119.
Henrichs, S. M. and W. S. Reeburgh. 1987. Anaerobic mineralization of marine sediment organic-
matter—Rates and the role of anaerobic processes in the oceanic carbon economy. Geomicrobiol.
J., 5, 191–237.
Hofman, P. A. G., S. A. Dejong, E. J. Wagenvoort, and A. J. J. Sandee. 1991. Apparent sediment
diffusion -coefficients fo r o xygen and oxyg en-con sumption rates measur ed with m icroelectr odes
and bell jars—Appli cation to oxyg en budge ts in es tuarine in tertidal se diments (Oos terscheld e,SW
Netherla nds). Mar. Ecol. Prog. Ser., 69, 261–272.
Jørgensen, B. B. 1977. The sulfur cycle of a coastal marine sediment (Limfjorden, Denmark).
Limnol. Ocean ogr., 22, 814–832.
—— 1982. Mineralization of organic matter in the sea bed—the role of sulfate reduction. Nature,
296, 643–645.
Jørgensen, B. B. and N. P. Revsbech. 1985. Diffusive boundary layer and the oxygen uptake of
sediment s and d etritus. Limnol. Oce anogr., 30, 111–122.
Lohse, L., E. H. G. Epping, W. Helder, W. v an Raa phorst. 1996. Oxygen pore water pro les in
continental shelf sediments of the North Sea: Turbulent versus molecular diffusion. Mar. Ecol.
Prog. Ser., 145, 63–75.
Lustwerk , R. L. and D. J. Burdige. 1 995. Elimina tion of diss olved sul d e interf erence in the ow
injectio n dete rmination o f SCO2by additio n of mo lybdate. Limno l. Ocean ogr., 40, 1011–1012.
Luther, G. W., P. J. Brendel, B. L. Lewis, B. Sundby, L. Lefrancois, N. Silverberg, D. B. Nuzzio.
1998. Simultaneous measurement of O2, Mn, Fe, I2, and S(2II) in marine pore waters with a
solid-s tate voltamme tric micro electrode .Limn ol. Oce anogr., 43, 325–333.
Mackin, J. E. and K. T. Swider. 1989. Organic matter decomposition pathways and oxygen
consump tion in co astal marin e sedimen ts. J. Mar. Res., 47, 681–716.
Marinelli, R. L. and B. P. Boudreau. 1996. An experimental and modeling study of pH and related
solutes i n an i rrigated ano xic coastal se diment. J. Mar. Res., 54, 939–966.
Martin, W. R. an d G. T. Bant a. 1992 . The mea surement of s ediment irr igation rate s: a compar ison of
the Br2tracer and 222Rn/226Ra dise quilibrium t echniqu es. J. Mar. Res., 50, 125–154.
Middelb urg, J. J., K. Soetaert and P. M. J. Herman . 1997. Empirical rela tionships for use in globa l
diagen etic models. Deep-Sea Res. I., 44, 327–344.
2000] 521
Furukawa et al.: Impact of burrow irrigation
Press, W. H., S. A. Teukolsky, W. T. Vetterling and B. P. Flannery. 1992. Numerical Recipes in
FORTRAN, Second Editio n. Cambridge Univ ersity Press, Cambri dge, 963 pp.
Rosenfeld, J. K. 1979. Interstitial water and sediment chemistry of two cores from Florida Bay. J.
Sed. Petrol., 49, 989–994.
Rude, P. D. and R. C. Aller. 1991. Fluorine mobility d uring early diagen esis of carbonate s ediment:
An indica tor of miner al transfo rmations. Geochim. Cos mochim. Acta, 55, 2491–2509.
Smethie, W. M., C. A. Nittrouer and R. F. L. Self. 1981. The use of radon-222 as a tracer of sediment
irrigat ion and mixi ng on the Washington c ontinen tal shelf. Mar. Geol., 42, 173–200.
Soetaert, K., P. M. J. Herman, and J. J. Middelburg. 1996. A model for early diagenetic processes
from the shelf to abyssal depths. Geoch im. Cosm ochim. Acta, 60, 1019–1040.
Stephens, K., P. Fleischer, D. L. Lavoie and C. Brunner. 1997. Scale-dependent physical and
geoacoustic property variability of recent, shallow-water carbonate sediments from the Dry
Tortugas Key s, Florida. Ge o-Marine Lett. , 17, 299–305.
Tromp, T. K., P. Van Cappellen and R. M. Key. 1995. A global model for the early diagenesis of
organic carbon and organic phosphorus in marine sediments. Geochim. Cosmochim. Acta, 59,
1259–1284.
Van Cappellen, P. and Y. Wang. 1996. Cycling of iron and manganese in surface sediments: Ageneral
theory for the coupled transport and reaction of carbon, oxygen, nitrogen, sulfur, iron, and
manganese. Amer. J. Sci., 296, 197–243.
Walter, L. M. a nd E. A. Burton. 1990 . Dissolutio n of rec ent platform carbo nate sediments in ma rine
pore uids. Amer. J. Sci., 290, 601–643.
Wang, Y. and P. Van Cappellen. 1996. A multicomponent reactive transport model of early
diagen esis: Applicatio n to redox cycling in c oastal marine s ediments . Geochim. Co smochim. Acta,
60, 2993–3014.
Wheatcroft, R. A., I. Olmez and F. X. Pink. 1994. Particle bioturbation in Massachusetts Bay:
Prelimina ry resu lts using a new d eliberate t racer t echniqu e. J. Mar. Res., 52, 1129–1150.
Wiltshire, K. H., F. Schroeder, H. D. Knauth, and H. Kausch. 1996. Oxygen consumption and
produc tion rates and associated uxes in sediment-water systems: A combination of mic roelec-
trode, incu bation and mode ling techniq ues.Arc h. Hydro biol., 137, 457–486.
Received: 15 June,1999; revised:30 March,2000.
522 Journal of Marine Research [58, 3
